(1) Technical Field
The present invention relates to a process for producing graphite films and fibers utilized in electrodes, heating elements, structures, gaskets for high-temperature and high-pressure instruments, heat-insulating materials, corrosion resistant sealing materials, brushes for electric use, X-ray monochromators and the like, particularly to a process for producing graphite films and fibers by heat treating a specific polymeric material as a starting material at a particular temperature.
(2) Background Information
Graphite holds an important position as an industrial material because of its outstanding heat resistance, chemical resistance, high electric conductivity and the like, and has been widely used as electrodes, heating elements and structures. Natural graphite may be used for such purposes. However, natural graphite occurs in an extremely limited amount and is intractable because of its powder or block form. Graphite has been, therefore, artificially produced.
Processes for producing such artificial graphite can be mainly classified into the following four processes.
In the first process, graphite is produced by separating from the melts of Fe, Ni/C system, decomposition of silicon carbide, aluminum carbide and so on, or cooling of the carbon melts under high temperature and high pressure. Graphite obtained in such manners is called Kish graphite, and has the same properties as those of natural graphite. According to this process, however, only a minute flake-like graphite is obtained. Therefore, together with the complexity of the manufacturing process and the expensive cost, this process has not been used for industrial production.
The second process is one in which various organic or carbonaceous materials are graphitized by heating at a temperature of at least 3000.degree. C. In this process, graphite having the same physical properties as those of natural graphite or Kish graphite can not be obtained. For example, natural graphite and Kish graphite have an electric conductivity in the direction of a-axis, which is the most typical property of graphite, of from 1.times.10.sup.4 S/cm to 2.5.times.10.sup.4 S/cm. In contrast to this, only the product having an electric conductivity of from 1.times.10.sup.3 S/cm to 2.times.10.sup.3 S/cm can generally be obtained by this process. That is to say, this fact shows that graphitization does not proceed well in this process. However, the products obtained by this second process have been widely employed in such uses where a perfect graphite is not necessarily required, because of the simplicity of the manufacturing process. Therefore, if graphite having the properties similar to those of natural graphite can be obtained by this process, it can be said that the industrial meaning thereof is significantly important.
In the third process, graphite is produced by high-temperature decomposition, sedimentation and hot working of gaseous hydrocarbons, wherein annealing is effected at a temperature of 3400.degree. C. under a pressure of 10 kg/cm.sup.2 for a long period of time. Graphite thus obtained is called highly orientated pyrographite and has almost the same properties as those of natural graphite. For example, it has an electric conductivity in the direction of a-axis of 2.5.times.10.sup.4 S/cm. According to this process, graphite of considerably large sizes can be prepared, unlike Kish graphite. This process has, however, disadvantages since the manufacturing process is complicated and the cost is expensive.
By the fourth process, natural graphite is immersed in a mixed solution of concentrated nitric acid and concentrated sulfuric acid, and thereafter the spacing between graphite layers is expanded by heating. Graphite thus obtained is called expanded graphite, and is powdery. Accordingly, it is further necessary to apply high-pressure press working to it in the presence of an adhesive, in order to make it sheet-like. Sheet-like graphite thus obtained is inferior to natural monocrystal graphite in properties. For example, the electric conductivity of sheet-like graphite is ordinary about 1.2.times.10.sup.3 S/cm. Further, a large amount of acids is required in this process. As a result, there are caused many problems such as generation of SO.sub.x and NO.sub.x gases, corrosion of metals due to exudation of residual acids.
As described hereinabove, the second and the fourth processes of the conventional processes 1 through 4 can not provide graphite having properties similar to those of natural monocrystal graphite. On the other hand, the first and the third processes can provide graphite having properties almost similar to those of natural monocrystal graphite, but have disadvantages that the processes are complicated and the products are highly expensive. The fourth process also contains many problems in the process.
The problems of the second process, which can be most easily conducted, will hereinafter be further described in detail. In this process, there are usually used as starting materials a carbonaceous material such as coke or the like and a binder such as pitch, coal tar or the like, and sometimes various polymers. However, perfect graphite can not be obtained from these starting materials as already described, even if they are heat treated at a temperature of about 3000.degree. C. For example, the electric conductivity of the product is usually in the range of 100 S/cm to 1000 S/cm, which is less than one-tenth that of perfect graphite.
With respect to the carbon structures produced at a temperature of about 3000.degree. C., various kinds of these structures exist, from one closely resembling the graphite structure to one remotely resembling it. Carbon which can be relatively easily converted to a graphitic structure by the mere heat treatment in this way is called graphitizable (soft) carbon, and carbon which is not so is called non-graphitizable (hard) carbon. The cause for such a difference in the structure is closely related to the mechanism of graphitization, and depends on whether the structural defects present in the carbon precursor are easily removed by the succeeding higher heat treatment or not. Of course, on the production of graphite films, a raw material from which graphitizable carbon is most easily produced is selected. However, it is difficult to remove completely the induced structural defects by heating. Accordingly, it has not hitherto been achieved to produce a graphite film which has the properties similar to those of natural monocrystal graphite by a mere heat treatment. Therefore, the fine structure of the carbon precursor plays an important role to the graphitizing property.
Against these processes using coke or the like as the starting material, some studies have been carried out to produce graphitic films by heat treating polymeric materials. It has been considered that these studies intend to control the fine structure of the carbon precursor while efficiently using the molecular structure of the polymeric material. In this process, the polymeric material is heat treated in vacuo or in an inert gas, and through decomposition and polycondensation reaction, the carbonaceous material is formed. In this case, however, graphitic films are not necessarily obtained from all of the polymeric materials used as the starting materials. Most of the polymeric materials can not be used for this purpose. The reason thereof will be explained as follows.
The reaction pathways of the polymeric compounds on heating are generally classified into three types, namely (1) gasification by random decomposition or depolymerization, (2) carbonization via pitch-like melts, and (3) carbonization while maintaining their solid state.
In the case of the reaction pathway (1), evaporation and gasification produce only a little carbonaceous material. It is apparent, therefore, that this type of polymers may not be suitable for graphitization.
Many of the materials which follow the reaction pathway (2) belong to a class of graphitizable materials. When they are merely heated in a non-oxidizing gas, however, they are lost to a great extent by evaporation and gasification. For this reason, in general, they are preliminarily heated in the presence of oxygen, to cross link the polymer chains to each other with oxygen, and thereafter carbonized or graphitized. At the same time, however, this causes the polymeric materials originally belonging to a class of graphitizable materials to be converted to non-graphitizable materials. Accordingly, films having a structure close to perfect graphite can not be obtained from the polymers preliminarily treated with oxygen, even if they are heat treated subsequently at a temperature of at least 3000.degree. C.
The reaction pathway (3), namely carbonization while maintaining the solid state, is most favorable from the viewpoint of the formation of the carbonaceous materials. However, most of the polymers which decompose through the pathway (3) are known to belong to a class of non-graphitizable materials and not to be capable of being converted to graphit films, even if they are heat treated at a temperature of at least 3000.degree. C. That is to say, for the polymeric materials which can form graphitic films, it is necessary to satisfy both requirements that they form the carbonaceous material by the heat treatment and that they belong to a class of graphitizable materials. As the polymers attempted to be heat treated for such a purpose, there are mentioned phenolformaldehyde resins, polyacrylonitrile, cellulose, polyimides, polyparaphenylenes, polyparaphenylene oxides, polyvinyl chloride and the like. Since all of them belong to a class of non-graphitizable materials, any product having a high degree of graphitization has not yet been obtained. The sole problem of the process for heat treating these polymers is how to find out such a polymeric material that easily forms the graphite film.